1. Field of the Invention
The present invention generally relates to image forming apparatuses having a recording head capable of discharging a droplet, and methods for correcting a landing position of the droplet discharged by the recording head.
2. Description of the Related Art
There are various types of image forming apparatuses, such as printers, facsimile machines, copy machines, plotters, and multifunction peripherals. One specific example is an inkjet recording apparatus of a liquid discharge recording type. This type of inkjet recording apparatus employs a recording head that discharges ink droplets onto a medium that is transported, in order to form (i.e., record, print, or transfer) an image or the like on the medium.
The term “medium” may refer not only to a sheet of paper but also an OHP (overhead projector) sheet or anything on which a droplet of ink or other liquid can attach. Such medium may be referred to by various names, such as a “recorded medium”, a “recording medium”, a “recording paper”, or a “recording sheet”.
There are two types of inkjet recording apparatus. One is called a serial-type image forming apparatus in which the recording head discharges droplets as it moves in a horizontal scan direction to form an image on the medium. The other is called a line-type image forming apparatus which employs a line-type recording head that remains stationary when it discharges droplets to form an image.
The terms “image forming apparatus” in the present disclosure are intended to refer to an apparatus for forming an image by discharging a liquid onto a medium of various material, such as paper, threads, fibers, fabrics, leather, metals, plastics, glass, wood, or ceramics. The terms “image formation” may refer not only to the imparting of an image with some meaning, such as a certain character or figure, onto a medium, but also to the imparting of an image without any meaning, such as a repetitive or random pattern, onto a medium. The term “ink” may refer to various liquids capable of forming an image on a medium, and may be referred to as a “recording fluid”, a “fixing solution”, and so on.
When a printing operation is carried out in an image forming apparatus of the liquid discharge type, a carriage on which the droplet-discharging recording head is mounted is moved back and forth in the horizontal scan direction in a reciprocal fashion. When characters or the like are printed in both the outward and the homeward directions of the movement of the carriage, a position error tends to be caused in ruled lines that are printed, between the outward path and the homeward path. Another problem is that an overlaid color error tends to be caused when different colors are laid one over another.
Some inkjet recording apparatuses may display a test chart for adjusting the landing position error, so that a user can select optimum values to adjust the discharge timing or the like. However, different users view such a test chart differently. A data input error may also be caused due to lack of experience in such an operation. As a result, the adjustment in the landing position error may actually lead to an increase in the adjustment error.
Japanese Laid-Open Patent Applications No. 4-39041 and 2005-342899, and Japanese Patent No. 3828251 disclose that a test pattern is formed on a transport belt or a medium-retaining transport member and the test pattern is read by a sensor.
Japanese Laid-Open Patent Application No. 2004-314361 discloses that a test pattern is formed on a recording paper and the test pattern is read by a sensor.
Japanese Laid-Open Patent Application No. 2007-152626 discloses that a linear encoder sensor is mounted on the carriage to read a linear encoder as the carriage is moved. Based on an output signal from the linear encoder sensor, the amount of movement of the carriage is measured by a position counter in order to determine a carriage position. Based on the detected carrier position, the landing position of ink droplets discharged out of the recording head on the recording media is shifted in the direction of movement of the carriage.
However, when the test pattern is formed on the transport belt or the medium and read by the sensor, it may be difficult to read the test pattern accurately depending on the combination of the color of the transport belt and that of the ink, such as when the color difference is very small. In order to detect the colors accurately, light sources with different wavelengths for different colors need to be used. In practice, however, such a test pattern formed on the transport belt cannot be read accurately by the conventional art.
For example, a conventional electrostatic adsorption belt is made of an insulating layer on the upper surface and an intermediate-resistance layer on the back surface in which carbon is mixed in order to provide electrical conductivity. In this case, the external color of the belt is black, so that a pattern formed on the belt cannot be distinguished from the black ink, thus failing to detect the pattern.
The present inventors have previously proposed a technology whereby a pattern made of independent ink droplets is formed on a pattern forming member having water repellency. The ink droplets are then irradiated with light of a single wavelength, and the amount of decrease of specular reflection due to the pattern is detected by utilizing the property of the ink droplets to form into hemispherical shapes. In this way, the position of the pattern and its position error are detected accurately.
When a transport belt is used as the water-repellent pattern forming member, for example, the surface condition of the transport belt changes depending on various factors, such as the condition of use and the number of times the pattern has been formed. Thus, the amount of decrease in specular reflection due to the formed pattern cannot be accurately measured when the surface condition of the transport belt is such that there is very little specular reflection therefrom.